The mechanisms of human sino-atrial and atrial arrhythmias will be further investigated in the cardiac catheterization laboratory by the use of intracardiac electrode catheters and coupled stimulation. Among the disturbances to be studied will be supraventricular ectopic plus reentrant A-V nodal and sino-atrial tachycardias, atrial fibrillation and atrial flutter. The effects of drugs on the institution, maintenance and resolution of these abnormalities will in particular be evaluated. The question of reentry versus automaticity as the mechanism of such arrhythmias and the location of the reentrant current will be investigated. With the use of atrial mapping, the route of abnormal tachycardias will be plotted. The effects of retrograde conduction on A-V nodal function in supra-ventricular arrhythmias wall be evaluated by the technique of coupled ventricular pacing. Particular attention will be paid to cases where sino-atrial and A-V nodal reentry coexist. Studies on ventricular excitability will also be continued. The effects of lidocaine and digoxin on this parameter of electrophysiological function will be determined. An effort will be made to uncover clues to the identification of those patients who are at particular risk of developing spontaneous and potentially fatal ventricular arrhythmias.